Effects of concentration and temperature on viscoelastic properties of aqueous potassium oleate solutions

The rheological properties of semidilute aqueous solutions containing long cylindrical micelles of an anionic surfactant, potassium oleate, are studied. It is shown that, at surfactant concentrations above 1 wt %, the rheological properties of the solutions are adequately described in terms of the simple Maxwell model of a viscoelastic liquid characterized by a single relaxation time. Based on the analysis of normalized dependences of the loss modulus on the storage modulus, the characteristic times of the processes governing the rheological properties of the above systems, i.e., the average breaking time and reptation time of micelles, are estimated. It is found that the breaking time of micelles decreases and relaxation time increases with increasing surfactant concentration due to lengthening of micellar chains.     

Investigation on the interaction between C16TAB and NaCMC in semidilute aqueous solution based on rheological measurement

The rheological behavior of unentangled and entangled semidilute solution of anionic polyelectrolyte sodium carboxymethyl cellulose (NaCMC) containing cationic surfactant cetyltrimethylammonium bromide (C₁₆TAB) was investigated. The results reveal that the rheological properties of these semidilute NaCMC solutions depend on the amount of C₁₆TAB added. In the unentangled semidilute NaCMC solution (0.5 g/L), the viscosity decreases with the increase of C₁₆TAB amount in the low surfactant concentration region (below the critical aggregation concentration, CAC). However, in high surfactant concentrations (above CAC), the viscosity decreases sharply with the increase in C₁₆TAB amount. It is found that viscosity change of NaCMC solution could be described using Colby’s model when surfactant concentrations are between CAC and saturated concentration (C_s), suggesting that no inter-polymer interaction exists between C₁₆TAB and NaCMC in the unentangled semidilute solution. However, for the entangled semidilute NaCMC solution (5 g/L), the addition of C₁₆TAB leads to an increase in viscosity. Meanwhile, the solution exhibits an enhanced shear thinning behavior due to adding more C₁₆TAB than 1 mM. The viscosity increase is ascribed to the physical cross linking of surfactant micelles with NaCMC chains. Furthermore, it is suggested that the enhanced shear thinning behavior results from weak interaction between NaCMC chains and C₁₆TAB micelles.     

Self-diffusion of polystyrene in solution 2. Discussion of experimental results on the basis of the reptation mechanism and entanglements

The expressions for polymer self-diffusion in semidilute solutions, theoretically derived from the reptation mechanism, the blob concept and scaling considerations, are discussed and compared against experimental data from the authors' investigations and the literature. In the nonentangled (from viscoelastic data) semidilute solution, the experimentally observed concentration and molar mass exponents are in fair agreement with those derived theoretically. However, a quantitative estimation shows that the experiments cannot be explained by reptation. Experiments with polymer mixtures also give strong evidence against reptation. It is concluded, that in the nonentangled semidilute solution, the polymer self-diffusion is more complicated than simple reptation. This is also supported by recently observed long-range density fluctuations or cluster formation in this concentration region detected by scattering techniques and NMR-PFGT. In the entangled semidilute solution, the self-diffusion data are in accordance with the reptation mechanism; reptation being within a tube having approximately 20 blobs between entanglements.     

Rheology of viscoelastic solutions of cationic surfactant. Effect of added associating polymer

Rheological studies were performed with aqueous salt solutions of viscoelastic cationic surfactant erucyl bis(hydroxyethyl)methylammonium chloride (EHAC) and its mixtures with hydrophobically modified polyacrylamide. The solutions of surfactant itself above the concentration of crossover of wormlike micelles exhibit two regions of rheological response. In the first region, they behave like polymer solutions in semidilute regime characterized by viscoelastic behavior with a spectrum of relaxation times. In the second region, unlike polymer solutions their relaxation after shear is dominated by a single relaxation time. Being composed of "living" micelles, the EHAC solutions easily lose their viscosity at the variation of the external conditions. For instance, heating from 20 to 60 degrees C reduces viscosity by up to 2 orders of magnitude, while added hydrocarbons induce a sudden drop of viscosity by 3-6 orders of magnitude. Polymer profoundly affects the rheological properties of EHAC solutions. The polymer/surfactant system demonstrates a 10,000-fold increase in viscosity as compared to pure-component solutions, the effect being more pronounced for polymer with less blocky distribution of hydrophobic units. A synergistic enhancement of viscosity was attributed to the formation of common network, in which some subchains are made up of elongated surfactant micelles, while others are composed of polymer. At cross-links the hydrophobic side groups of polymer anchor EHAC micelles. In contrast to surfactant itself, the polymer/surfactant system retains high viscosity at elevated temperature; at the same time it keeps a high responsiveness to hydrocarbon medium inherent to EHAC.     

Influence of chain charge and complexation on the overlap and entanglements formation in poly(acrylic acid) salt-containing aqueous solutions

Dilute-semidilute regime crossover in aqueous solutions of partly neutralized poly(acrylic acid) and of its complex with tetradecyltrimethylammonium bromide was studied by light scattering and viscometry methods. The chain charge growth causes the decrease of overlap concentration (c*) and the increase of the entanglements formation concentration (ce), hence, the semidilute unentangled regime of solution expands. Complexation of the polyelectrolyte with an oppositely charged surfactant leads to c* increase and to ce decrease. It is shown that in semidilute entangled solutions the surfactant acts as an effective structuring agent because of the binding of polyelectrolyte chains via surfactant micelles.     

Molecular dynamics simulations of supramolecular polymer rheology

Using equilibrium and nonequilibrium molecular dynamics simulations, we studied the equilibrium and rheological properties of dilute and semidilute solutions of head-to-tail associating polymers. In our simulation model, a spontaneous complementary reversible association between the donor and the acceptor groups at the ends of oligomers was achieved by introducing a combination of truncated pseudo-Coulombic attractive potential and Lennard Jones repulsive potential between donor, acceptor, and neighboring groups. We have calculated the equilibrium properties of supramolecular polymers, such as the ring/chain equilibrium, average molecular weight, and molecular weight distribution of self-assembled chains and rings, which all agree well with previous analytical and computer modeling results. We have investigated shear thinning of solutions of 8- and 20-bead associating oligomers with different association energies at different temperatures and oligomer volume fractions. All reduced viscosity data for a given oligomer length can be collapsed into one master curve, exhibiting two power-law regions of shear-thinning behavior with an exponent of -0.55 at intermediate ranges of the reduced shear rate β and -0.8 (or -0.9) at larger shear rates. The equilibrium viscosity of supramolecular solutions with different oligomer lengths and associating energies is found to obey a power-law scaling dependence on oligomer volume fraction with an exponent of 1.5, in agreement with the experimental observations for several dilute or semidilute solutions of supramolecular polymers. This implies that dilute and semidilute supramolecular polymer solutions exhibit high polydispersity but may not be sufficiently entangled to follow the reptation mechanism of relaxation.     

Effect of temperature on the structure of adsorption layers formed by adsorption of polymers in the transition region from dilute to semidilute solutions

The measurements of adsorption from solutions of polymers (poly(butyl methacrylate) and polystyrene) and their mixtures at various temperatures in the wide concentration region from dilute to semidilute solution have been made. The adsorption isotherms and fraction of bound segments confirm the existence of the transition concentration region near the critical concentration of the chain overlapping when the change of adsorption mechanisms take place. The effect of temperature on adsorption from the mixtures is different for both the polymers depending on the concentration regime. For PBMA, which adsorbs from the mixtures preferentially, the adsorption isotherms are of the same shape as for adsorption from binary solutions and are characterized by the presence of a small forepart in the transition region and by increasing adsorption with temperature. For less adsorbing PS by transition from dilute to semidilute regime the inversion of the temperature effect on adsorption is observed. These results are confirmed by the estimations of the parameters of preferential adsorption at various temperatures. The data on the fraction of bound segments for dilute regime corresponds to the extended conformation of chains at the surface. The transition to semidilute regime leads to the diminishing of the fraction of bound segments as a result of simultaneous adsorption of macromolecular aggregates. Values of adsorption layer thickness have been calculated for various solution regimes and concentration. The dependence of the adsorption layer thickness on the temperature and on the solution regime at which adsorption occurs was established.     

Wormlike micelles mediated by polyelectrolyte

Aqueous solutions of the anionic surfactant potassium oleate (K-oleate) were studied using small-angle neutron scattering (SANS), steady-state rheology, and cryogenic transmission electron microscopy (cryo-TEM). The micellar structural changes induced by the addition of potassium chloride (KCl) and sodium polystyrenesulfonate (PSS) of different molecular weights were investigated. Upon addition of KCl, a transition from spherical to wormlike micelles was detected from the SANS data and confirmed by the cryo-TEM pictures. The rheological measurements revealed a strong dependence of the low-shear viscosity on the concentration of salt: a broad maximum in the viscosity curve was observed upon addition of KCl, characteristic of the growth of micelles into long worms, followed by branching. The addition of PSS to salt-free solutions of K-oleate had a significant effect on the scattering patterns, revealing partial growth of the spherical micelles into rodlike micelles. In contrast, in the presence of high salt concentrations, addition of PSS to solutions of wormlike micelles did not bring any noticeable modifications in the scattering. However, in the same salt conditions, a clear effect was observed on the low shear viscosity upon addition of PSS, which was found to depend significantly on molecular weight. This suggests a novel way of impacting the viscosity of solutions of wormlike micelles.     

Self-aggregation of mixtures of oppositely charged polyelectrolytes and surfactants studied by rheology, dynamic light scattering and small-angle neutron scattering

In this study, the phase behavior, structure and properties of systems composed of the cationic, cellulose-based polycation JR 400 and the anionic surfactants sodium dodecylbenzenesulfonate (SDBS) or sodium dodecylethoxysulfate (SDES), mainly in the semidilute regime, were examined. This system shows the interesting feature of a very large viscosity increase by nearly 4 orders of magnitude as compared to the pure polymer solution already at very low concentrations of 1 wt%. By using rheology, dynamic light scattering (DLS), and small-angle neutron scattering (SANS), we are able to deduce systematic correlations between the molecular composition of the systems (characterized by the charge ratio Z=[+(polymer)]/[?(surfactant)]), their structural organization and the resulting macroscopic flow behavior. Mixtures in the semidilute regime with an excess of polycation charge form highly viscous network structures containing rodlike aggregates composed of surfactant and polyelectrolyte that are interconnected by the long JR 400 chains. Viscosity and storage modulus follow scaling laws as a function of surfactant concentration (η~c(s)(4); G(0)~c(s)(1.5)) and the very pronounced viscosity increase mainly arises from the strongly enhanced structural relaxation time of the systems. In contrast, mixtures with excess surfactant charges form solutions with viscosities even below those of the pure polymer solution. The combination of SANS, DLS, and rheology shows that the structural, dynamical, and rheological properties of these oppositely charged polyelectrolyte/surfactant systems can be controlled in a systematic fashion by appropriately choosing the systems composition.     

Capillary breakup extensional rheometry of sodium carboxymethylcellulose solutions in water and propylene glycol/water mixtures

This article presents the results of capillary break‐up extensional rheometer experiments conducted for semidilute solutions of carboxymethylcellulose sodium salt (Na‐CMC) with degrees of substitution (DS) ranging from 0.62 to 1.04 in distilled water and propylene glycol (PG)/water mixtures. The partial aggregation of Na‐CMC chains with DS < 1 observed in aqueous solutions triggers an increase in apparent extensional viscosity and extension of break‐up time. The rheological properties of Na‐CMC solutions in propylene glycol/water mixture are determined by the solubility of the polymer and the physical crosslinking of chains. The disappearance of the elasto‐capillary regime during the filament thinning of Na‐CMC solutions with DS < 1 in propylene glycol/water mixture was linked to the physical crosslinking of polymer chains. The shape of the extensional viscosity curve for Na‐CMC solutions with DS = 1.04 in PG/water mixture was characteristic for semidilute polymer solutions with a low number of entanglements. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019 , 57, 1537–1547     

Mutual macromolecular crowding as the basis for polymer solution non‐ideality

Semidilute polymer solutions differ greatly from dilute solutions in properties such as viscosity, relaxation time, elastic modulus, colloid osmotic pressure, and light scattering. Previously, Matsuoka and Cowman proposed a single semiempirical expression for the nonideality contribution due to the concentration and intrinsic viscosity dependence, which has no other adjustable parameters, but quantitatively fits data for flexible, semiflexible, and rigid polymers in good solvents. In this report, the excluded volume theory as proposed by Ogston and Laurent is generalized to include mutual crowding between identical polymers based on hydrodynamic volumes, and applied to derive the expression for the nonideality contribution to specific viscosity, colloid osmotic pressure, and light scattering. Additionally, consideration of the contribution of mutual macromolecular crowding to the effective solvent viscosity allows prediction of polymer relaxation time and elastic modulus in semidilute solutions. This theoretical approach now allows the prediction of semidilute polymer solution properties based only on concentration and intrinsic viscosity, and conversely allows intrinsic viscosity (and thus average molecular weight) to be calculated from measurements made on semidilute solutions of known concentration. Copyright © 2016 John Wiley & Sons, Ltd.     

Interaction between a nonionic surfactant and a hydrophobically modified 2-hydroxyethyl cellulose

Interaction between the nonionic surfactant Tergitol 15-S-7 and hydrophobically modified 2-hydroxyethyl cellulose (HMHEC) was studied rheologically in a semidilute regime of HMHEC. The low-shear viscosity of HMHEC was increased with addition of surfactant from 25 to 250 ppm, in which the critical micelle concentration of surfactant was near 39 ppm, and then decreased to a value smaller than that of pure HMHEC with further addition of surfactant to 1000 ppm. An interesting shear-induced phenomenon was observed. The steady-state shear measurements show that there exist crossovers between viscosity-shear rate curves of HMHEC solutions with and without surfactant added, whereas it was not observed in the HEC-surfactant systems. Moreover, added Tergitol 15-S-7 reversed the temperature effect on the viscosity of the HMHEC solution. That is, increasing temperature to or near the cloud point raises the viscosity of the HMHEC-surfactant aggregates, in contrast to the viscosity decrease in the pure HMHEC solutions. A possible mechanism based on the necklace model and the clouding phenomenon is conjecturally introduced to explain such phenomena.     

The microrheology of polystyrene sulfonate combs in aqueous solution

Video particle tracking (VPT) and diffusing wave spectroscopy were used to characterize the microrheology of polystyrene sulfonate combs in aqueous solutions. At low frequencies VPT demonstrated predominantly viscous behavior. The manner in which the viscosity scaled as a function of monomer concentration was a sensitive function of the comb architecture. Densely branched combs or combs with long side chains demonstrated entangled polyelectrolyte scaling above the overlap concentration, whereas sparsely branched combs had unentangled polyelectrolyte scaling. A dynamic scaling model was developed for the viscosity of unentangled semidilute solutions of comb polyelectrolytes. Diffusing wave spectroscopy demonstrated Rouse modes (G' approximately G" approximately omega12) for the high-frequency dynamics of the semidilute comb solutions. The form of the high-frequency viscoelasticity was independent of the chain architecture and the modulus scaled as expected for linear flexible polyelectrolytes.     

Effect of polymer on rheological behavior of heated solutions of potassium oleate cylindrical micelles

The rheological characteristics of aqueous solutions of potassium oleate cylindrical micelles and their mixtures with hydrophobized polyacrylamide are studied at different temperatures and polymer concentrations no higher than the concentration of overlapping of coils. It is shown that, at all temperatures, the viscosities of surfactant-polymer solutions appear to be noticeably higher than the viscosities of individual surfactant solutions; however, the presence of the polymer has no effect on the viscous flow activation energy.     

DISPERSION OF CHOLESTEROL IN AQUEOUS SURFACTANT SOLUTIONS: INTERPRETATION OF VISCOSITY DATA

The viscosity of cholesterol dispersions in aqueous buffered surfactant solutions has been reported under progel conditions. The viscosity versus concentration curves pass through maximum which corresponds to the beginning of solubilization of cholesterol into the surfactant solutions. The stability of the dispersions has been explained by a mechanism involving formation of association complex between cholesterol and the surfactants through hydrogen bonding.     

Temperature dependence of the non-Newtonian viscosity of elongated micellar solutions

We report in this work new results of the study on the non-Newtonian viscosity of aqueous micellar solutions of cetyltrimethylammonium bromide (CTAB) in the presence of potassium bromide (KBr), in the concentration range where the elongated micelles overlap. The experiments have been performed as a function of the surfactant concentration, temperature and shear rate by use of a Couette-viscosimeter.In the non-Newtonian range, at relatively low surfactant concentration (0.25 M/l), our results show that the flow curves obtained at different temperatures converge to a single liner curve with a characteristic slope varying with the surfactant concentration. These same data can be superposed on a master curve when appropriate reduced variables are used. The shape of the flow curves obtained at different temperatures for a sufficiently high surfactant concentration is similar to that obtained for monodisperse polymer solutions at different molecular weights. The slope obtained of about –1 is also predicted by Graessley's model in the theory of microviscoelasticity based on the concept of entanglement for polymer solutions. However, at surfactant concentration higher than 0.25 M/l our results show an unusual behavior. Above some critical shear rate it is possible to obtain an increase of the apparent viscosity with temperature. One possible explanation of this effect can be found in the increase of the entanglement with concentration coupled with the temperature and direct now effects on scission and recombination rate of the micelles.     

Microemulsions of Potassium Oleate,Tween-20, Propylene Carbonate,and Ethylene Glycol as Drug Vehicles for Mefenamic Acid

Different microemulsions were prepared with and without mefenamic acid (MFA). The base microemulsion was mainly composed of distilled water; the aqueous phase, propylene carbonate; the oil phase, potassium oleate; the surfactant, and finally di-ethylene glycol; the cosurfactant. The effect of mixing ionic (potassium oleate) with nonionic (Tween-20) surfactant was investigated via constructing the phase diagrams of such systems. Changes in conductivity and viscosity of the freshly prepared microemulsion over time were monitored as an indication for the stability of the microemulsion. Measurements were carried out at room temperature, after a freeze-thaw cycle and also after storage for 3 days at 60°C, where the latter is treated as an accelerated test for the time-temperature effects on the stability of a microemulsion. It was found that a set of surfactants, instead of a single surfactant, and inclusion of cosurfactant resulted in a broader region where a stable microemulsion is predominant. At a mass ratio of 1:2 of potassium oleate to Tween-20, O/W microemulsions were found to have maximum stability among all examined systems, under the accelerated test, such that they have a minimum portion of combined surfactants and cosurfactant of 60 wt% and maximum of 80 wt%. With the aforementioned specifications, no phase separation and neither significant change in the conductivity nor in the viscosity was observed in any of the examined systems after subjecting them both to the accelerated and freeze-thaw cycle test, indicating that such systems were thermodynamically stable. Samples of micro emulsions passing previous tests were further subjected to an acidic medium by dispersing 1 g of MFA-containing microemulsion in 10 g HCl solution (pH 1) in a shaking water bath at 37°C, for a 6 hour period. The maximum solubility of MFA in a stable microemulsion was approximately 5 wt%, evaluated at room temperature.     

Self-assembled networks highly responsive to hydrocarbons

Rheological studies were performed with aqueous salt solutions of anionic surfactant potassium oleate and its mixtures with hydrophobically modified polyacrylamide. Semidilute solutions of the surfactant in the presence of salt (KCl) demonstrate viscoelastic properties due to the formation of a transient network of entangled wormlike micelles. These systems are highly responsive to hydrocarbons: the addition of n-heptane or n-dodecane reduces the viscosity of solutions by up to 4 to 5 orders of magnitude, thus inducing the transition of a gellike system to a fluid one. It is the transformation of cylindrical surfactant micelles into spherical ones upon absorption of hydrocarbon that disrupts the network. The addition of a small amount (0.5 wt %) of associating polymer leads to up to a 5000-fold increase in the zero-shear viscosity and enhances the susceptibility to hydrocarbons. SANS data show that independently of the presence of polymer the radius of wormlike micelles is roughly equal to the length of a surfactant molecule, whereas the radius of spheres formed upon the absorption of hydrocarbon is 2-2.5-fold higher. A possible structure of the spherical micelles is discussed.     

Anionic wormlike micellar fluids that display cloud points: rheology and phase behavior

We report our investigations into the self-assembly of sodium oleate (NaOA) in the presence of a binding salt (triethylammonium chloride, Et(3)NHCl) simple salt (potassium chloride, KCl). Both salts promote the growth of long, wormlike micelles in NaOA solutions, thereby increasing the fluid viscosity. The significant difference with the Et(3)NHCl salt is that it also modifies the phase behavior of NaOA solutions. Specifically, NaOA/Et(3)NHCl solutions display cloud points upon heating, followed by phase separation into two liquid phases. Such cloud point behavior is rarely observed in ionic surfactant systems, although it is common in nonionic surfactant solutions. Interestingly, while cloud points are not observed with KCl, the addition of KCl to NaOA/Et(3)NHCl solutions further lowers the cloud point temperature. Also, in the case of tetraethylammonium halide salt, neither a cloud point nor an increase in viscosity is observed. The clouding in the case of Et(3)NHCl is attributed to the temperature-induced aggregation of anionic micelles whose surface is covered by bound counterions.     

Properties of semidilute polymer solutions: Investigation of an optically labeled three-component system

A three-component system containing a polymer (2), a good solvent (1) for that polymer, and a second polymer (3) that is compatible with component (2) and isorefractive with the solvent (1) has been studied by static and dynamic light-scattering methods. In concentrated toluene (1) solutions of poly(vinyl methyl ether) (3), where appreciable chain overlap occurs and excluded-volume effects are reduced, polystyrene (2) may be studied in the dilute-solution limit. Consequently, these light-scattering measurements provide an explicit measure of both thermodynamic and hydrodynamic changes that occur as the total polymer concentration is increased from dilute to concentrated solution. Precise numerical coefficients, correct scaling exponents, the radius of gyration, and the effective hydrodynamic radius can be measured directly along with the observation of long-wave single-chain reptation motions and short-range cooperative motions in semidilute and concentrated solutions.